Stormwater drainage system is designed to drain excess rain and ground water from impervious surfaces such as paved streets, car parks, parking lots, footpaths, sidewalks, and roofs. Stormwater drains vary in design from small residential dry wells to large municipal systems. Many stormwater drainage systems are designed to drain the untreated stormwater containing pollutants into sea, lakes or rivers. In the stormwater drainage system, stormwater is collected to a flow-through stormwater sump or well. The stormwater sump is generally a concrete cylinder having an inlet pipe and an outlet pipe located at distance from the bottom of the sump. The particles suspended in stormwater are supposed to settle as a sediment layer on the bottom of the sump. Under low flow conditions sump is able to remove suspended sediment, but under high flow conditions the sump is unable to retain the particles when the water enters the sump at high flow rate. Then the sump becomes filled with water and the incoming high rate flow creates an undesirable circular flow pattern inside the sump which causes resuspension and wash out of the sediment from the bottom to the outlet pipe. In prior art, documents U.S. Pat. Nos. 8,715,507, 8,663,466 and US 2012/0255622 have proposed to retrofit the sump with a baffle. The baffle has proved to be efficient to reduce the undesirable wash out of the sediment. U.S. Pat. No. 8,663,466 and US 2012/0255622 disclose stormwater drain baffles having a plurality of openings distributed over the area of the baffle. The baffle comprises at least two baffle portions connected to each other. The baffle portions are attached to a support frame structure which is rather complicated having many different parts.